The present invention relates to probe assemblies of the type commonly used for testing integrated circuits (IC) and, in particular, the present invention relates to a membrane probing assembly having contacts which scrub, in a locally controlled manner, across the respective input/output conductors of each device so as to reliably wipe clear the surface oxides that are normally found on those conductors thereby ensuring good electrical connection between the probing assembly and each device.
The trend in electronic production has been toward increasingly smaller geometries particularly in integrated circuit technology wherein a very large number of discrete circuit elements are fabricated on a single substrate or "wafer." After fabrication, this wafer is divided into a number of rectangular-shaped chips or "dice" where each die presents a rectangular or other regular arrangement of metallized contact pads through which input/output connections are made. Although each die is eventually packaged separately, for efficiency sake, testing of the circuit formed on each die its preferably performed while the dies are still joined together on the wafer. One typical procedure is to support the wafer on a flat stage or "chuck" and to move the wafer in X, Y and Z directions relative to the head of the probing assembly so that the contacts on the probing assembly move from die to die for consecutive engagement with each die. Respective signal, power and ground lines are run to the probing assembly from the test instrumentation thus enabling each circuit to be sequentially connected to the test instrumentation.
One conventional type of probing assembly used for testing integrated circuits provides contacts that are configured as needle-like tips. These tips are mounted about a central opening formed in a probe card so as to radially converge inwardly and downwardly through the opening. When the wafer is raised beyond that point where the pads on the wafer first come into contact with these tips, the tips flex upwardly so as to skate forwardly across their respective pads thereby removing oxide buildup on the pads.
The problem with this type of probing assembly is that the needle-like tips, due to their narrow geometry, exhibit high inductance so that signal distortion is large in high frequency measurements made through these tips. Also, these tips can act in the manner of a planing tool as they wipe across their respective pads, thereby leading to excessive pad damage. This problem is magnified to the extent that the probe tips bend out of shape during use or otherwise fail to terminate in a common plane which causes the more forward ones of the tips to bear down too heavily on their respective pads. Also, it is impractical to mount these tips at less than 100 micron center-to-center spacing or in a multi-row grid-like pattern so as to accommodate the pad arrangement of more modern, higher density dies. Also, this type of probing assembly has a scrub length of the needle tips of 25 microns or more, which increases the difficulty of staying within the allowed probing area.
In order to reduce inductive losses, decrease pad wear and accommodate smaller device geometries, a second type of probing assembly has been developed that uses a flexible membrane structure for supporting the probing contacts. In this assembly, lead lines of well-defined geometry are formed on one or more plies of flexible insulative film, such as polyimide or MYLAR.TM.. If separate plies are used, these plies are bonded together to form, for example, a multilayered transmission line structure. In the central portion of this flexible structure or membrane, each conductive line is terminated by a respective probing contact which is formed on, and projects outwardly from, an outer face of the membrane. These probing contacts are arranged in a predetermined pattern that matches the pattern of the device pads and typically are formed as upraised bumps for probing the flat surfaces conventionally defined by the pads. The inner face of the membrane is supported on a supporting structure. This structure can take the form, for example, of a truncated pyramid, in which case the inner face of the center portion of the membrane is supported on the truncated end of the support while the marginal portions of the membrane are drawn away from the center portion at an angle thereto so as to clear any upright components that may surround the pads on the device.
With respect to the membrane probing assembly just described, excessive line inductance is eliminated by carefully selecting the geometry of the lead lines, and a photolithographic process is preferably used to enable some control over the size, spacing, and arrangement, of the probing contacts so as to accommodate higher density configurations. However, although several different forms of this probing assembly have been proposed, difficulties have been encountered in connection with this type of assembly in reducing pad wear and in achieving reliable clearing of the oxide layer from each of the device pads so as to ensure adequate electrical connection between the assembly and the device-under-test.
One conventional form of membrane probing assembly, for example, is exemplified by the device shown in Rath European Patent Pub. No. 259,163A2. This device has the central portion of the sheet-like membrane mounted directly against a rigid support. This rigid support, in turn, is connected by a resilient member comprising an elastomeric or rubber block to the main body of the assembly so that the membrane can tilt to match the tilt of the device. Huff U.S. Pat. No. 4,918,383 shows a closely related device wherein radially extending leaf springs permit vertical axis movement of the rigid support while preventing it from tilting so that there is no slippage or "misalignment" of the contact bumps on the pads and further so that the entire membrane will shift slightly in the horizontal plane to allow the contacts to "scrub" across their respective pads in order to clear surface oxides from these pads.
In respect to both of these devices, however, because of manufacturing tolerances, certain of the contact bumps are likely to be in a recessed position relative to their neighbors and these recessed bumps will not have a satisfactory opportunity to engage their pads since they will be drawn away from their pads by the action of their neighbors on the rigid support. Furthermore, even when "scrub" movement is provided in the manner of Huff, the contacts will tend to frictionally cling to the device as they perform the scrubbing movement, that is, there will be a tendency for the pads of the device to move in unison with the contacts so as to negate the effect of the contact movement. Whether any scrubbing action actually occurs depends on how far the pads can move, which depends, in turn, on the degree of lateral play that exists as a result of normal tolerance between the respective bearing surfaces of the probe head and chuck. Hence this form of membrane probing assembly does not ensure reliable electrical connection between each contact and pad.
A second conventional form of membrane probing assembly is exemplified by the device shown in Barsotti European Patent Pub. No. 304,868A2. This device provides a flexible backing for the central or contact-carrying portion of the flexible membrane. In Barsotti, the membrane is directly backed by an elastomeric member and this member, in turn, is backed by a rigid support so that minor height variations between the contacts or pads can be accommodated. It is also possible to use positive-pressure air, negative-pressure air, liquid or an unbacked elastomer to provide flexible backing for the membrane, as shown in Gangroth U.S. Pat. Nos. 4,649,339, Ardezzone 4,636,772, Reed, Jr. et al. 3,596,228 and Okubo et al. 5,134,365, respectively. These alternative devices, however, do not afford sufficient pressure between the probing contacts and the device pads to reliably penetrate the oxides that form on the pad surfaces.
In this second form of membrane probing assembly, as indicated in Okubo, the contacts may be limited to movement along the Z-axis in order to prevent slippage and resulting misalignment between the contacts and pads during engagement. Thus, in Barsotti, the rigid support underlying the elastomeric member is fixed in position although it is also possible to mount the support for Z-axis movement in the manner shown in Huff U.S. Pat. No. 4,980,637. Pad damage is likely to occur with this type of design, however, because a certain amount of tilt is typically present between the contacts and the device, and those contacts angled closest to the device will ordinarily develop much higher contact pressures than those which are angled away. The same problem arises with the related assembly shown in European Patent Pub. No. 230,348A2 to Garretson, even though in the Garretson device the characteristic of the elastomeric member is such as to urge the contacts into lateral movement when those contacts are placed into pressing engagement with their pads. Yet another related assembly is shown in Evans U.S. Pat. No. 4,975,638 which uses a pivotably mounted support for backing the elastomeric member so as to accommodate tilt between the contacts and the device. However, the Evans device is subject to the friction clinging problem already described insofar as the pads of the device are likely to cling to the contacts as the support pivots and causes the contacts to shift laterally.
Yet other forms of conventional membrane probing assemblies are shown in Crumly U.S. Pat. Nos. 5,395,253, Barsotti et al. 5,059,898 and Evans et al. 4,975,638. In Crumly, the center portion of a stretchable membrane is resiliently biased to a fully stretched condition using a spring. When the contacts engage their respective pads, the stretched center portion retracts against the spring to a partially relaxed condition so as to draw the contacts in radial scrub directions toward the center of the membrane. In Barsotti, each row of contacts is supported by the end of a respective L-shaped arm so that when the contacts in a row engage their respective pads, the corresponding arm flexes upwardly and causes the row of contacts to laterally scrub simultaneously across their respective pads. In both Crumly and Barsotti, however, if any tilt is present between the contacts and the device at the time of engagement, this tilt will cause the contacts angled closest to the device to scrub further than those angled further away. Moreover, the shorter contacts will be forced to move in their scrub directions before they have had the opportunity to engage their respective pads due to the controlling scrub action of their neighboring contacts. A further disadvantage of the Crumly device, in particular, is that the contacts nearer to the center of the membrane will scrub less than those nearer to the periphery so that scrub effectiveness will vary with contact position.
In Evans et al. U.S. Pat. No. 5,355,079 each contact constitutes a spring metal finger, and each finger is mounted so as to extend in a cantilevered manner away from the underlying membrane at a predetermined angle relative to the membrane. A similar configuration is shown in Higgins U.S. Pat. No. 5,521,518. It is difficult, however, to originally position these fingers so that they all terminate in a common plane, particularly if a high density pattern is required. Moreover, these fingers are easily bent out of position during use and cannot easily be rebent back to their original position. Hence, certain ones of the fingers are likely to touch down before other ones of the fingers, and scrub pressures and distances are likely to be different for different fingers. Nor, in Evans at least, is there an adequate mechanism for tolerating a minor degree of tilt between the fingers and pads. Although Evans suggests roughening the surface of each finger to improve the quality of electrical connection, this roughening can cause undue abrasion and damage to the pad surfaces. Yet a further disadvantage of the contact fingers shown in both Evans and Higgins is that such fingers are subject to fatigue and failure after a relatively low number of "touchdowns" or duty cycles due to repeated bending and stressing.
Referring to FIG. 1, Cascade Microtech, Inc. of Beaverton, Oregon has developed a probe head 40 for mounting a membrane probing assembly 42. In order to measure the electrical performance of a particular die area 44 included on the silicon wafer 46, the high-speed digital lines 48 and/or shielded transmission lines 50 of the probe head are connected to the input/output ports of the test instrumentation by a suitable cable assembly, and the chuck 51 which supports the wafer is moved in mutually perpendicular X,Y,Z directions in order to bring the pads of the die area into pressing engagement with the contacts included on the lower contacting portion of the membrane probing assembly.
The probe head 40 includes a probe card 52 on which the data/signal lines 48 and 50 are arranged. Referring to FIGS. 2-3, the membrane probing assembly 42 includes a support element 54 formed of incompressible material such as a hard polymer. This element is detachably connected to the upper side of the probe card by four Allen screws 56 and corresponding nuts 58 (each screw passes through a respective attachment arm 60 of the support element, and a separate backing element 62 evenly distributes the clamping pressure of the screws over the entire back side of the supporting element). In accordance with this detachable connection, different probing assemblies having different contact arrangements can be quickly substituted for each other as needed for probing different devices.
Referring to FIGS. 3-4, the support element 54 includes a rearward base portion 64 to which the attachment arms 60 are integrally joined. Also included on the support element 54 is a forward support or plunger 66 that projects outwardly from the flat base portion. This forward support has angled sides 68 that converge toward a flat support surface 70 so as to give the forward support the shape of a truncated pyramid. Referring also to FIG. 2, a flexible membrane assembly 72 is attached to the support after being aligned by means of alignment pins 74 included on the base portion. This flexible membrane assembly is formed by one or more plies of insulative sheeting such as KAPTON.TM. sold by E. I. Du Pont de Nemours or other polyimide film, and flexible conductive layers or strips are provided between or on these plies to form the data/signal lines 76.
When the support element 54 is mounted on the upper side of the probe card 52 as shown in FIG. 3, the forward support 66 protrudes through a central opening 78 in the probe card so as to present the contacts which are arranged on a central region 80 of the flexible membrane assembly in suitable position for pressing engagement with the pads of the device under test. Referring to FIG. 2, the membrane assembly includes radially extending arm segments 82 that are separated by inwardly curving edges 84 that give the assembly the shape of a formee cross, and these segments extend in an inclined manner along the angled sides 68 thereby clearing any upright components surrounding the pads. A series of contact pads 86 terminate the data/signal lines 76 so that when the support element is mounted, these pads electrically engage corresponding termination pads provided on the upper side of the probe card so that the data/signal lines 48 on the probe card are electrically connected to the contacts on the central region.
A feature of the probing assembly 42 is its capability for probing a somewhat dense arrangement of contact pads over a large number of contact cycles in a manner that provides generally reliable electrical connection between the contacts and pads in each cycle despite oxide buildup on the pads. This capability is a function of the construction of the support element 54, the flexible membrane assembly 72 and their manner of interconnection. In particular, the membrane assembly is so constructed and connected to the support element that the contacts on the membrane assembly preferably wipe or scrub, in a locally controlled manner, laterally across the pads when brought into pressing engagement with these pads. The preferred mechanism for producing this scrubbing action is described in connection with the construction and interconnection of a preferred membrane assembly 72a as best depicted in FIGS. 6 and 7a-7b.
FIG. 6 shows an enlarged view of the central region 80a of the membrane assembly 72a. In this embodiment, the contacts 88 are arranged in a square-like pattern suitable for engagement with a square-like arrangement of pads. Referring also to FIG. 7a, which represents a sectional view taken along lines 7a--7a in FIG. 6, each contact comprises a relatively thick rigid beam 90 at one end of which is formed a rigid contact bump 92. The contact bump includes thereon a contacting portion 93 which comprises a nub of rhodium fused to the contact bump. Using electroplating, each beam is formed in an overlapping connection with the end of a flexible conductive trace 76a to form a joint therewith. This conductive trace in conjunction with a back-plane conductive layer 94 effectively provides a controlled impedance data/signal line to the contact because its dimensions are established using a photolithographic process. The backplane layer preferably includes openings therein to assist, for example, with gas venting during fabrication.
The membrane assembly is interconnected to the flat support surface 70 by an interposed elastomeric layer 98, which layer is coextensive with the support surface and can be formed by a silicone rubber compound such as ELMER'S STICK-ALL.TM. made by the Borden Company or Sylgard 182 by Dow Corning Corporation. This compound can be conveniently applied in a paste-like phase which hardens as it sets. The flat support surface, as previously mentioned, is made of incompressible material and is preferably a hard dielectric such as polysulfone or glass.
In accordance with the above-described construction, when one of the contacts 88 is brought into pressing engagement with a respective pad 100, as indicated in FIG. 7b, the resulting off-center force on the rigid beam 90 and bump 92 structure causes the beam to pivot or tilt against the elastic recovery force provided by the elastomeric pad 98. This tilting motion is localized in the sense that a forward portion 102 of the beam moves a greater distance toward the flat support surface 70 than a rearward portion 104 of the same beam. The effect is such as to drive the contact into lateral scrubbing movement across the pad as is indicated in FIG. 7b with a dashed-line and solid-line representation showing the beginning and ending positions, respectively, of the contact on the pad. In this fashion, the insulative oxide buildup on each pad is removed so as to ensure adequate contact-to-pad electrical connections.
FIG. 8 shows, in dashed line view, the relative positions of the contact 88 and pad 100 at the moment of initial engagement or touchdown and, in solid-line view, these same elements after "overtravel" of the pad by a distance 106 in a vertical direction directly toward the flat support surface 70. As indicated, the distance 108 of lateral scrubbing movement is directly dependent on the vertical deflection of the contact 88 or, equivalently, on the overtravel distance 106 moved by the pad 100. Hence, since the overtravel distance for each contact on the central region 80a will be substantially the same (with differences arising from variations in contact height), the distance of lateral scrubbing movement by each contact on the central region will be substantially uniform and will not, in particular, be affected by the relative position of each contact on the central region.
Because the elastomeric layer 98 is backed by the incompressible support surface 70, the elastomeric layer exerts a recovery force on each tilting beam 90 and thus each contact 93 to maintain contact-to-pad pressure during scrubbing. At the same time, the elastomeric layer accommodates some height variations between the respective contacts. Thus, referring to FIG. 9a, when a relatively shorter contact 88a is situated between an immediately adjacent pair of relatively taller contacts 88b and these taller contacts are brought into engagement with their respective pads, then, as indicated in FIG. 9b, deformation by the elastomeric layer allows the smaller contact to be brought into engagement with its pad after some further overtravel by the pads. It will be noted, in this example, that the tilting action of each contact-is locally controlled, and the larger contacts are able, in particular, to tilt independently of the smaller contact so that the smaller contact is not urged into lateral movement until it has actually touched down on its pad.
Referring to FIGS. 10 and 11, the electroplating process to construct such a beam structure, as schematically shown in FIG. 8, includes the incompressible material 68 defining the support surface 70 and the substrate material attached thereon, such as the elastomeric layer 98. Using a flex circuit construction technique, the flexible conductive trace 76a is then patterned on a sacrificial substrate. Next, a polyimide layer 77 is patterned to cover the entire surface of the sacrificial substrate and of the traces 76a, except for the desired location of the beams 90 on a portion of the traces 76a. The beams 90 are then electroplated within the openings in the polyimide layer 77. Thereafter, a layer of photoresist 79 is patterned on both the surface of the polyimide 77 and beams 90 to leave openings for the desired location of the contact bumps 92. The contact bumps 92 are then electroplated within the openings in the photoresist layer 79. The photoresist layer 79 is removed and a thicker photoresist layer 81 is patterned to cover the exposed surfaces, except for the desired locations for the contacting portions 93. The contacting portions 93 are then electroplated within the openings in the photoresist layer 81. The photoresist layer 81 is then removed. The sacrificial substrate layer is removed and the remaining layers are attached to the elastomeric layer 98. The resulting beams 90, contact bumps 92, and contacting portions 93, as more accurately illustrated in FIG. 12, provides the independent tilting and scrubbing functions of the device.
Unfortunately, the aforementioned construction technique results in a structure with many undesirable characteristics.
First, several beams 90, contact bumps 92, and contacting portions 93 (each of which may be referred to as a device) proximate one another results in different localized current densities within the electroplating bath, which in turn results in differences in the heights of many of the beams 90, contact bumps 92, and contacting portions 93. Also, different densities of the ions within the electroplating bath and "random" variations in the electroplating bath also results in differences in heights of many of the beams 90, contact bumps 92, and contacting portions 93. The different heights of many of the beams 90, contact bumps 92, and contacting portions 93 is compounded three fold in the overall height of many of the devices. Accordingly, many devices will have a significantly different height than other devices. Using membrane probes having variable device height requires more pressure to ensure that all the contacting portions 93 make adequate contact with the test device than would be required if all the devices had equal overall height. For high density membrane probes, such as 2000 or more devices in a small area, the cumulate effect of the additional pressure required for each device may exceed the total force permitted for the probe head and probe station. The excess pressure may also result in bending and breaking of the probe station, the probe head, and/or the membrane probing assembly. In addition, the devices with the greatest height may damage the pads on the test device because of the increased pressure required to make suitable contact for the devices with the lowest height.
Second, the ability to decrease the pitch (spacing) between the devices is limited by the "mushrooming" effect of the electroplating process over the edges of the polyimide 77 and photoresist layers 79 and 81. The "mushrooming" effect is difficult to control and results in a variable width of the beams 90, contact bumps 92, and contacting portions 93. If the height of the beams 90, the contact bumps 92, or the contacting portions 93 are increased then the "mushrooming" effect generally increases, thus increasing the width of the respective portion. The increased width of one part generally results in a wider overall device which in turn increases the minimum spacing between contacting portions 93. Alternatively, decreasing the height of the beams 90, the contact bumps 92, or the contacting portions 93 generally decreases the width of the "mushrooming" effect which in turn decreases the minimum spacing between contacting portions 93. However, if the height of the contacting portions 93 relative to the respective beam 90 is sufficiently reduced, then during use the rearward end of the beam 90 may sufficiently tilt and contact the test device in an acceptable location, i.e., off the contact pad.
Third, it is difficult to plate a second metal layer directly on top of a first metal layer, such as contacting portions 93 on the contact bumps 92, especially when using nickel. To provide a bond between the contact bumps 92 and the contacting portions 93, an interface seed layer such as copper or gold is used to make an improved interconnection. Unfortunately, the interface seed layer reduces the lateral strength of the device due to the lower sheer strength of the interface layer.
Fourth, applying a photoresist layer over a non-uniform surface tends to be semi-conformal in nature resulting in a non-uniform thicknesses of the photoresist material itself. Referring to FIG. 13, the photoresist layer 79 (and 81) over the raised portions of the beams 90 tends to be thicker than the photoresist layer 79 (and 81) over the lower portions of the polyimide 77. In addition, the thickness of the photoresist 79 (and 81) tends to vary depending on the density of the beams 90. Accordingly, regions of the membrane probe that have a denser spacing of devices, the photoresist layer 79 (and 81) will be thicker on average than regions of the membrane probe that have a less dense spacing of devices. During the exposing and etching processing of the photoresist layer 79 (and 81), the duration of the process depends on the thickness of the photoresist 79 (or 81). With variable photoresist. thickness it is difficult to properly process the photoresist to provide uniform openings. Moreover, the thinner regions of photoresist layer 79 (or 81) will tend to be overexposed resulting in variably sized openings. Also, the greater the photoresist layer thickness 79 (or 81) the greater the variability in its thickness. Accordingly, the use of photoresist presents many processing problems.
Fifth, separate alignment processes are necessary to align the beams 90 on the traces 76a, the contact bumps 92 on the beams 90, and the contacting portions 93 on the contact bumps 92. Each alignment process has inherent variations that must be accounted for in sizing each part. The minimum size of the contacting portions 93 is defined primarily by the lateral strength requirements and the maximum allowable current density therein. The minimum size of the contacting portions 93, accounting for the tolerances in alignment, in turn defines the minimum size of the contact bumps 92 so that the contacting portions 93 are definitely constructed on the contact bumps 92. The minimum size of the contact bumps 92, in view of the contacting portions 93 and accounting for the tolerances in alignment, defines the minimum size of the beams 90 so that the contact bumps 92 are definitely constructed on the beams 90. Accordingly, the summation of the tolerances of the contact bumps 92 and the contacting portions 93, together with a minimum size of the contacting portions 93, defines the minimum device size, and thus defines the minimum pitch between contact pads.
What is desired, therefore, is a membrane probe construction technique and structure that results in a more uniform device height, decreased spacing between devices, maximized lateral strength, desired geometries, and proper alignment.